The manufacturing costs of integrated circuits are largely dependent upon the chip area required to implement desired functions. The chip area, in turn, is defined by the geometries and sizes of the active components such as gate electrodes in metal-oxide-semiconductor (MOS) technology, and diffused regions such as MOS source and drain regions and bipolar emitters and base regions. These geometries and sizes are often dependent upon the photolithographic resolution available for the particular manufacturing facility. The goal of photolithography in establishing the horizontal dimensions of the various devices and circuits is to create a pattern which meets design requirements as well as to correctly align the circuit pattern on the surface of the wafer. As line widths shrink smaller and smaller in submicron photolithography, the process to print lines and contact holes in photoresist becomes increasingly more difficult.
With circuit advancement to the very-large-scale integration (VLSI) levels, more and more layers are added to the surface the wafer. These additional layers in turn create more steps on the wafer surface. The resolution of small image sizes in photolithography thus becomes more difficult over the additional steps due to light reflection and the thinning of the photoresists over the steps. Planarization techniques are generally incorporated to offset the effects of a varied topography.
The fabrication of a circuit starts with a blank silicon substrate. The processing of the substrate requires a substantial number of manufacturing steps. A number of approaches are used in the art to make the surface of the different portions of the integrated circuit more smooth. These approaches will make it easier to provide smooth and well-formed subsequent layers. These techniques are generally referred to as planarization.
The thickness of the various layers are dictated by a number of considerations, such as providing a reasonable margin of insulation between conductive layers, or for metal layers, providing sufficient current-carrying capacity. Since the layers cannot be made arbitrarily thin, it is necessary to do something to make the top surface of lower layers more smooth (or more planar) in order to provide a proper base for subsequent layers.
In the prior art, various spin-on-glass (SOG) processes have been used to help planarize interlevel dielectric layers. These processes, however, have proven inadequate for today's sub-micron technologies. One problem in the prior art is the poor quality of filling of small space geometries. This poor filling is caused, in part, by local SOG volumetric limitations as well as surface tension effects which tend to drag the SOG out of filled spaces during the spinning process.
It is therefore an object of this invention to provide a method of improving the planarization of the surface of the integrated circuit.
It is a further object of this invention to improve planarization through the use of two SOG layers, preferably having different etch rates.
It is a further object of this invention to provide such a method which utilizes conventional process flows.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.